FOSSILS, the impressions or remains of plants or animals preserved in rocks by natural causes. Fossils supply data, from which the geologist can determine the relative ages of sedimentary rocks. The first man to realize their chronological importance was the English surveyor, William Smith, known as the father of historical geology, although acute observers like Leonardo da Vinci had pointed out long before that fossil shells were not freaks of nature, but had been laid down where found in the sediments of some body of water.

Most fossil remains are of marine types; many are of fresh-water and land-and-water types; comparatively few are of land types. The reason is plain. Animal remains lying on the ground are eaten by animals, or if not eaten, soon decay, and the bony skeleton, if buried by sand or loam, is slowly dissolved by percolating water. Plant remains decay even more rapidly. In water, decay is retarded. Thus mastodon remains have been found in swamps where the animals were occasionally mired, but of the infinitely greater number of mastodons that died on drier ground no trace is left. Old lake beds are frequently rich in plant and animal remains. Fishes, insects, birds and land animals, and the leaves, flowers and fruits of trees are preserved in the fine-grained shales or the sands and clays of the lake deposits in several Western States. It is evident in many cases that the animals have been drifted into the lake by rivers; and in some cases large deposits of disjointed skeletons of many species are found jumbled together. In old swamps plants formed thick masses of vegetable matter, now turned to coal, imprints of leaves and stems being common in the shale overlying a coal-seam. In the sea, conditions are most favorable for the preservation of organic remains, and marine deposits have formed thick and extensive beds. Hence, of all the fossils found, marine types are most numerous. Fossils are preserved in several ways, which may be classified under four heads. (1) Some of the original substance may be preserved, as the carbon in a leaf, or in the bone or shell of some animal. (2) All the original substance may dissolve away, but its shape may be preserved. This may happen in two ways: (a) the external form may be preserved in the sediments, forming a mold, or (b) the internal form may be preserved, forming a cast. A mollusk dies; its soft parts decay; the interior of the shell fills with sand or ooze; the shell is deeply buried. The sediments consolidate; the calcareous material of the shell may dissolve; but both its external and internal form are preserved. (3) Less commonly the structure of organic remains is preserved by a true petrifaction, the organic substance being replaced, atom by atom, by some mineral compound, like silica or calcium carbonate. A striking illustration of this method of preservation is fossil wood, in which the replacing silica preserves minute details of structure. (4) In certain circumstances animals of long past ages may be preserved unchanged wholly or in part by a fortunate accident. The Siberian mammoths and woolly rhinoceroses frozen into perpetual ice, and the insects protected from decay in the Baltic amber and other ancient resins, are good examples, elsewhere described.

The study of fossils (palæozoology, palæobotany) has carried our knowledge of the life of the globe back to its beginning, and illustrated by actual examples the steps in its evolution. It has also, by means of chemistry and the microscope, taught zoologists many new things in respect to anatomy and histology. Finally, the delicate art of extracting fossils from their matrix, and preparing them for intelligent use, has developed a technique that amounts to a regular industry in connection with museums. See Coal; Coral Islands; Geology; Paleontology; Paleobotany.